EP2586057B1 - Thermogenerator comprising phase-change materials - Google Patents

Description

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a thermogenerator using phase change materials, and more generally to a thermoelectric power source.

By thermoelectric effect, the appearance of a potential difference at the junction of two conductive materials of different types subjected to a temperature difference is referred to as this effect is also known as the Seebeck effect.

The most well-known use of the Seebeck effect is temperature measurement using thermocouples.

Until now, thermoelectric conversion systems or thermoelectric sources for generating electricity from heat have been confined to niche markets because of their low yields and high costs.

In the case of an environment in which the temperature is most often strictly homogeneous, the system is not subjected to a temperature gradient continuously. It is then necessary to provide an electric battery to store the generated current, when a temperature gradient appears. As a result, power generation is subject to events that are not under control.

Moreover, in the case of autonomous systems, i.e. not connected to a device for controlling the temperature difference, the temperature difference is fluctuating and does not allow the constant generation of voltage and current. This is particularly troublesome in the case of a power supply of a battery, which does not support the variation of voltage at its terminals.

The document EP 1 001 470 describes a wristwatch powered by a thermoelectric element. Two phase change material elements may be provided. In the intended arrangement and in the intended operation, it is not possible to control the thermal gradient within the thermoelectric element so that it is constant over a given period of time and the thermoelectric element delivers a stable electrical power .

US 2003/143958 discloses a device in which a thermoelectric generator is disposed between an electronic component and a phase change material.

It is therefore an object of the present invention to provide a thermogenerator employing phase change materials, for which the generation of a stable electrical power is controlled.

STATEMENT OF THE INVENTION

The previously stated purpose is achieved by a thermogenerator having at least one thermoelement and two phase change material elements having different phase change temperatures, the phase change material elements being disposed on either side of the thermoelement so that it only sees the temperature gradient imposed by the two phase change material elements. Thus, when each of the phase change material elements is composed of both a liquid phase and a solid phase, it is at a constant temperature. Therefore, the temperature gradient across the thermoelement is constant. The thermoelement is then able to deliver a stable electrical power.

In other words, the phase change material elements and the thermoelement are thermally arranged in series, i.e. the thermoelement is therefore subject only to the temperatures of the two phase change material elements.

It is sought to deliver a constant thermal power to the thermoelement, which implies a constant temperature gradient since all the other parameters are already constant.

The phase change material elements are disposed on both sides of the thermoelement so that they are fully subjected to the temperatures of the phase change material.

Particularly advantageously, it is possible to provide thermal short-circuits between the two phase-change materials in order to lengthen the period during which the temperature gradient across the thermoelement is constant, in the melting phase. These thermal short-circuits can be realized by fluidic communication between the phase-change material elements, causing a premature appearance of a seed of liquid in the phase change material not directly subject to the heat source. Connections between the phase change material tanks are made to rapidly create a seed appearance in the least exposed material and lengthen the period during which the temperature is constant.

Also particularly advantageously, one can provide areas in which the solidification is delayed to extend the period during which the temperature gradient is constant, in the solidification phase. For this, there is provided an area in which the thickness of the phase change material is greater than the distance of the melting front.

The present invention therefore relates to a thermogenerator comprising at least one thermoelement and two phase change materials having different phase change temperatures, said at least one thermoelement having two opposite main faces, each of said faces being covered by one of the materials. phase change so that the thermoelement only sees the temperature gradient imposed by the two phase change materials, during a heating or cooling phase.

In an exemplary embodiment, the thermogenerator comprises enclosures containing the phase-change materials, each enclosure comprising a first and a second part, the first part comprising a flat plate, one face of which is provided with projecting fins, and the second part comprising a part comprising a flat plate, one side of which is in contact with the thermoelement and the other side is provided with projecting fins, the two first and second parts being mounted face to face so that the fins interpenetrate defining a cavity in which is the phase change material.

k étant la conduction thermique du matériau à changement de phase, L étant la chaleur latente de fusion du matériau à changement de phase, ΔT étant la différence de température entre la température de la paroi de l'ailette et la température de changement de phase du matériau à changement de phase, et t étant le temps.Advantageously, the thickness of phase-change material in at least a portion of each of the enclosures is of the order and, preferably, slightly less than or equal to the distance of the melting front, the distance of the melting front being equal to $\sqrt{\frac{2.k\mathrm{.}\mathrm{\Delta }T\mathrm{.}t}{\mathrm{The}}},$

where k is the heat conduction of the phase change material, L is the latent heat of fusion of the phase change material, where ΔT is the temperature difference between the fin wall temperature and the phase change temperature of the phase change material, and t being the time.

Particularly advantageously, the thermogenerator according to the present invention comprises means for directly contacting the two phase change materials.

Said means for directly contacting the two phase-change materials may comprise at least one conduit connecting an enclosure in which the first phase-change material is located and an enclosure in which the second phase-change material is located, said duct being partially filled by the first phase change material and the second phase change material.

k étant la conduction thermique du matériau à changement de phase, L étant la chaleur latente de fusion du matériau à changement de phase, ΔT étant la différence de température entre la température de paroi du conduit et la température de changement de phase du matériau à changement de phase, et t étant le temps.Preferably, the transverse dimension of the duct is less than or equal to the distance from the melting front, the distance from the melting front being equal to $\sqrt{\frac{2.k\mathrm{.}\mathrm{\Delta }T\mathrm{.}t}{\mathrm{The}}},$

where k is the heat conduction of the phase change material, L is the latent heat of fusion of the phase change material, where ΔT is the temperature difference between the duct wall temperature and the phase change temperature of the change material. phase, and t being the time.

According to a preferred embodiment of the invention, the transverse dimension of the duct is much smaller than the distance from the melting front, for example, representing only 0.01 to 50% of this distance, or else from 0.1 to 20. %.

The conduit may be of a material having good thermal conductivity, that is to say a material whose thermal conductivity is greater than that of the MCP used in the invention.

For example the conduit may be aluminum, copper steel or stainless steel.

Advantageously, a central portion of the duct is made of a material having a limited thermal conductivity, that is to say whose thermal conductivity is lower than that of phase change materials.

For example, said portion of the conduit may be glass or plastic material.

It can be provided that at least one volume of liquid separates the first and second phase change materials in said conduit.

Also particularly advantageously, the chamber containing the phase charging material having the highest phase change temperature comprises a zone of phase change material whose thickness is greater than the distance of the melting front.

This can be achieved by providing that the distance between two fins is locally greater than the distance from the fusion front. Alternatively, it is expected that the enclosure having an insert tank containing phase change material, the transverse dimension of said tank being greater than the distance of the melting front.

Preferably, the thermogenerator according to the invention is surrounded by thermal insulation means to guide the thermal flow through the stack formed by the phase change materials and the moiris a thermoelement.

The present invention also relates to a power generation system comprising a thermogenerator according to the present invention and a heat source.

The heat source may be disposed on the side of the enclosure containing the phase change material having the highest phase change temperature.

In another embodiment, the electricity generation system according to the intention may comprise a closed fluidic circuit, containing a heat transfer fluid, said circuit being able to exchange heat with the first and second phase-change material, and passing through the heat source, in which the heat source is located downstream phase change material having the lowest phase change temperature and upstream of the phase change material having the highest phase change temperature in the coolant flow direction, and wherein when the coolant exchanges with the phase change material having the lowest phase change temperature, its temperature is at least equal to the phase change temperature of said material, and when the coolant exchanges heat with the change material phase with the highest phase change temperature, its temperature is at least equal to the change temperature phase of said material.

The heat source is for example formed by at least one integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une représentation schématique d'un thermogénérateur selon la présente invention,
• la figure 2A est une représentation schématique d'un exemple de réalisation pratique d'un thermogénérateur selon la présente invention comportant des courts-circuits thermiques entre les deux matériaux à changement de phase,

• la figure 2B est une vue de détail de la figure 2A,
• les figures 3A à 3C sont des vues de détail de la figure 2A au niveau d'un court-circuit thermique dans différents états,
• les figures 4A à 4C sont des vues d'une variante de réalisation d'un court-circuit thermique dans différents états,
• la figure 5 est une représentation schématique d'un système de génération d'électricité comportant thermogénérateur selon la présente invention associé à un circuit fluidique,
• la figure 6 est une représentation graphique de l'évolution de l'énergie stockée ou, libérée dans les matériaux à changement de phase en fonction de la température.

The present invention will be better understood with the aid of the description which will follow and the appended drawings, in which:

• the figure 1 is a schematic representation of a thermogenerator according to the present invention,
• the Figure 2A is a schematic representation of an example of practical realization of a thermogenerator according to the present invention comprising thermal shorts between the two phase change materials,

• the Figure 2B is a detail view of the Figure 2A ,
• the FIGS. 3A to 3C are detailed views of the Figure 2A at the level of a thermal short circuit in different states,
• the Figures 4A to 4C are views of an alternative embodiment of a thermal short circuit in different states,
• the figure 5 is a schematic representation of an electricity generating system comprising a thermogenerator according to the present invention associated with a fluid circuit,
• the figure 6 is a graphical representation of the evolution of stored or released energy in phase change materials as a function of temperature.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

On the figure 1 , a schematic representation of a thermogenerator according to the present invention comprising a thermoelement 2 and first and second elements 4, 6 each containing a phase change material MCP1, MCP2 can be seen. In the remainder of the description, the phase-change materials will be referred to as "MCP1 material" and "MCP2 material".

The thermoelement 2 is provided with two opposite faces 8, 10 of larger areas. Each of the faces 8, 10 is in contact with one of the phase change material elements 4, 6.

The first and second elements 4, 6 with phase change material completely cover the two faces 8, 10 of the thermoelement 2.

The first and second elements 4, 6 with phase change material each comprise an enclosure 12, 14 provided with a face 12.1, 14.1 in contact with a face 8, 10 of the thermoelement 2, and the MCP1 material, MCP2 disposed in the enclosure 12, 14.

Advantageously, the thermogenerator is surrounded by a thermal insulator 17 guiding the thermal flow along the axis of the stack and also reducing heat loss to the outside.

According to the invention, the MCP1, MCP2 materials have different solid-liquid phase change temperatures T _f1 , T _f2 .

The MCP1 material has the highest melting temperature and is intended to be disposed on the side of a heat source 15. This heat source may be an integrated circuit of a computer, a photovoltaic cell, a solar thermal component, an electronic power component such as an insulated gate bipolar transistor or an IGBT transistor ("Insulated Gate Bipolar Transistor"), a microelectronic component such as a microprocessor ...

The term "thermoelement" designates any device capable of generating electrical power when it is subjected to a temperature gradient.

For example, the thermoelement 2 comprises a substrate and one or more connected PN junctions serial. The PN junctions are formed by an N-doped semiconductor material and a P-doped semiconductor material. The materials are alternately arranged and extend between the two faces 8, 10 of the thermoelement. Interconnections are provided between the N-doped materials and the adjacent P-doped materials so as to form the PN junctions. The two materials of a PN junction thus see the same thermal flux imposed by the materials MCP1 and MCP2. PN junctions are electrically connected in series.

The materials of the PN junctions are separated by the substrate, which is chosen to be an electrical insulator to prevent electrical short-circuiting of the PN junctions and to be a good thermal insulator to prevent a thermal short circuit between MCP1 material and MCP2 material. The substrate may be for example flexible polymer, ceramic or metal. The polymer used may be a thermosetting polymer.

The voltage ΔV at the terminals of the module is a function of the number of P-N junctions, the larger the voltage, the higher the voltage.

The connection between the N-doped materials and the P-doped materials and the interconnections between the P-N junctions are, for example, made of copper.

The MCP1 material has a phase change temperature T1, it imposes the temperature T1 on the face 8 of the thermoelement and the face 10 of the thermoelement is at the temperature of the material MCP2 T ₂ .

It is the difference ΔT = T1 - T2 which is responsible for the appearance of a voltage ΔV at the terminals of the thermoelectric module 2.

avec

• ΔV la différence de potentiel en Volt aux bornes du module thermoélectrique,
• ΔT le gradient de température en °C au niveau des jonctions P-N,
• S le coefficient Seebeck en V.K^-1.

The Seebeck effect is expressed by the following relation: $$\mathrm{.DELTA.V}=\mathrm{DT}\mathrm{.}\mathrm{S}$$

with

• ΔV the potential difference in Volt across the thermoelectric module,
• ΔT the temperature gradient in ° C at the PN junctions,
• S the Seebeck coefficient in VK ^-1 .

Thus, the higher the temperature gradient ΔT imposed, the higher the potential difference ΔV generated by the thermogenerator.

The value of ΔT is chosen as a function of the desired power, for example if a power of a few hundred milliwatts is desired, a temperature difference ΔT of 20 ° C. suffices. In the case of a desired power of a few watts, a temperature difference of several hundred ° C is required.

When heat is supplied by the heat source, the phase change materials progressively change from a solid state to a liquid state, the liquid being confined in the chamber.

The thermoelement 2 is then subjected to a temperature gradient between its first face 8 and its second face. The phase change material elements form a thermal buffer between the heat source and the thermoelement on the one hand and opposite back of the thermoelement on the other hand, and thus impose a thermal gradient to the thermoelement.

On the figure 6 the variation of the stored energy and the released energy E as a function of the temperature T for the two materials MCP1, MCP2 can be represented. The energy variation of the MCP1 material is shown in solid lines, and that of MCP2 in dashed lines. The reference S designates the solid phase alone, the reference L designates the liquid phase alone, and the reference S + L the mixture of the liquid phase and the solid phase.

Each of the materials MCP1, MCP2 have constant temperatures T _f1 , T _f2 as long as a solid phase and a liquid phase coexist. These periods are represented by the vertical lines on the figure 6 .

The zone designated ΔTcst during which the two phase change materials are simultaneously at constant temperatures T _f1 , T _f2 is delimited by the horizontal solid lines. It is this period that allows the generation of a stable electrical power.

According to the present invention, the thermoelement only sees the temperature gradient imposed by the two phase change materials MCP1, MCP2. Consequently, during the period when the two materials MCP1, MCP2 are at constant temperatures T _f1 , T _f2 , the temperature gradient seen by the thermoelement 2 is constant. The thermogenerator can then produce a stable electrical power.

Thanks to the invention, it is then easy to obtain through phase change materials the generating a stable power from a heat source.

On the Figures 2A and 2B a particularly advantageous embodiment of a thermogenerator according to the present invention can be seen providing elongated periods of generation of stable electrical power.

The references used for the figure 1 will be used to designate the same elements on the Figure 2A .

This embodiment differs from that of the figure 1 , in particular in that the thermogenerator comprises means 16 forming thermal short circuits between the first phase change material element 4 and the second phase change material element 6.

In addition, on the Figure 2A an example of a practical embodiment of the phase change material elements 4, 6 and more particularly the enclosures can be seen.

In this example, each chamber comprises an outer wall 19, for example glass, Pyrex® type and means for transferring heat between the phase-change material, and the thermoelement. The thermal conductivity of the enclosure is low and preferably lower than that of the PCM.

In the example shown, the means for transferring heat comprises an outer part 18 and an inner part 20 nested one inside the other. The parts 18, 20 comprise a plate 18.1 and fins 22 projecting from one of the faces of the plate 18.1. The parts are made of a material with good thermal conductivity, such as aluminum or copper.

The fins 22 have for example a triangular section. The fins 22 have the function of developing an upper heat exchange surface between the phase change material and the external environment. The parts are characterized by their "surface development" factor which is equal to the ratio of the finned workpiece surface to the non-finned workpiece surface. For example, a development factor of 2 makes it possible to double the storage power density of the phase change material. Preferably, we will choose parts presenting. On the Figure 2A the part development factor is of the order of 6.

The two parts 18, 20 are nested one inside the other so that a fin 22 of a part 18, 20 is received between two fins 22 of the other part 20, 18. These two parts 18, 20 then define a cavity 24 having a zigzag section. The flat face of the plate 18.1 of the outer part 18 of the first phase change material element 4 receives the heat flux F of the heat source 15, and the flat face of the inner part 20 of the first material element to change phase 2 and the flat face of the inner part 20 are in contact with the thermoelement 2.

The fins 22 of the inner parts 20 form fins leading the heat of the material MCP1 to thermoelement 2, then thermoelement 2 to MCP2 material.

In the case of the phase change material element 4 located on the heat source side 15, the fins 22 of the outer part 18 also serve to conduct heat from the heat source 15 to the MCP1 material.

The inner parts 20 are for example glued on the faces of the thermoelement 2 to ensure good thermal conduction.

The assembly between the outer wall and the two parts 18, 20 is sealed to the phase change materials by conventional means known to those skilled in the art.

The MCP1 material is for example RT58® whose phase change temperature is 58 ° C and the MCP2 material is RT35H® whose phase change temperature is 35 ° C from Rubitherm®.

For the sake of simplicity of construction, the two enclosures are identical, however, concerning the enclosure containing the material MCP2, it could not include fins 22 at the outer part 18, since it is not desired drive the heat towards the outside of the thermogenerator.

Advantageously, the thickness of the phase change materials is substantially constant in each of the enclosures. This thickness corresponds to the distance separating the facing faces of two adjacent fins 22, which forms two surfaces heat exchange with the phase change material.

The average thickness of phase change material corresponds to a characteristic length Lc.

Particularly advantageously, the characteristic length Lc is of the order of, and preferably less than or equal to the distance traveled by the melting front. By the order of, we mean that we tolerate a difference of plus or minus 30%. Thus, the heat transmitted by the fins 22 can traverse the entire thickness of the material and melt it completely in a given period of time depending on the heat source.

avec :

• k : la conduction thermique,
• L : la chaleur latente de fusion du MCP,
• ΔT : la différence de température entre la température de la paroi des ailettes et la températures de changement de phase du MCP,
• t : le temps.

The distance of the fusion front is given by the following relation: $$\mathrm{Distance\; from\; the\; melting\; front}=\sqrt{\frac{2.k\mathrm{.}\mathrm{\Delta }T\mathrm{.}t}{\mathrm{The}}}$$

with:

• k: thermal conduction,
• L: the latent heat of fusion of the MCP,
• ΔT: the temperature difference between the fin wall temperature and the phase change temperature of the MCP,
• t: the time.

The storage power of a phase change material is given by the following relationship $$\mathrm{Power\; storage}=\sqrt{\frac{\mathrm{The}\mathrm{.}k\mathrm{\Delta }T}{2t}}$$

Therefore, the distance of the melting front depends on the storage power and the time.

In the table below, different distance values of the melting front are grouped according to the storage power and the time, considering that the MCP1 and MCP2 materials have a thermal conductivity equal to 0.2 W / m / K and the heat source is at a temperature of 100 ° C.

Depending on the operating conditions of the thermogenerator, the advantageous maximum thickness of the phase change materials in the enclosures can be calculated.

It is understood that it is in no case necessary that the two materials have the same thermal conductivity. We can predict that they are different, in this case the thicknesses of the two phase change materials in the two speakers would be different.

We will describe in detail the means 16 of thermal short-circuit.

On the FIGS. 3A to 3C an example of such thermal short circuit means 16 can be seen in three different states.

In this example, the thermal short-circuit means 16 comprise one or more conduits 26 connecting the inside of the reservoir of the material MCP1 to the reservoir of the material MCP2. In the example shown, the thermogenerator comprises two ducts 26. When the materials MCP1 and MCP2 are in the solid state, the portion of the duct on the side of the enclosure containing the material MCP1, upper part in the representation of the Figure 2A , is filled with MCP1 material, and the portion of the conduit on the side of the enclosure containing the material MCP2, lower part in the representation of the Figure 2A , is filled with MCP2 material. A volume 28 filled with gas, for example air, is provided between the two materials MCP1, MCP2 in the solid state.

Advantageously, the conduit 26 is made of a material offering good thermal conductivity, for example it is made of metal, for example aluminum or copper.

Even more advantageously, the central portion of the duct extending between at least the MCP1-air or liquid interface material and at least the air or liquid interface and MPC2 material, has a low thermal conductivity so that the majority of the heat, for example of the order of at least 90%, passes through the phase change materials and not through the wall of the duct. For example, this portion of conduit is made of a material having a lower thermal conductivity than phase change materials, for example such as a plastic material or a glass and / or have a reduced thickness to provide a reduced section for heat flow. For example, this central portion can represent between 10% and 30% of the total length of the conduit.

The thermogenerator according to this embodiment may also include several conduits 26 distributed between the enclosures containing the materials MCP1, MCP2.

The thermogenerator may also comprise several ducts arranged next to each other.

When the MCP1 material melts, it expands and comes into contact with the MCP2 material to which it transmits heat, causing it to melt. MCP1 and MCP2 materials are chosen to be immiscible.

Particularly advantageously, the transverse dimension of the duct 26, ie its diameter in the case of a circular section duct, is less than the characteristic dimension Lc, in order to quickly obtain a complete melting of the phase-change material MCP1 in the conduit, especially in its center. The distance of the fusion front corresponds to the distance traveled by the fusion front during a given time. By choosing the diameter of the duct less than this distance, the melting front will have traveled the diameter of the tube, it is then certain that all the MCP1 material in the pipe is melted.

The examples of dimensions given previously also apply for the dimensioning of the ducts. Thus, depending on the operating conditions of the thermogenerator, can calculate the maximum advantageous diameter for the duct connecting the two enclosures.

The ratio of the volume of MCP2 material contained in the duct and the volume to the volume of MCP2 material in the chamber is advantageously less than 30%, preferably less than 20%, more preferably of the order of 2 to 15%. .

We will now explain the operation of this thermogenerator, considering that the MCP1, MCP2 materials are solid.

When the MCP1 phase change material element is subjected to a heat source, the phase change material MCP1 begins to melt, the MCP1 material in the conduit 26 also begins to melt ( figure 3A ). During the melting, the material MCP1 is at the constant temperature T _f1 . It then comes into contact with the MCP2 material located in the duct ( figure 3B ). There is then transmission of a small amount of heat between the material MCP1 and the material MCP2 in the conduit, the latter melts ( figure 3C ). The MCP2 material is then at the constant temperature T _f2 .

Thus, thanks to these means 16 thermal short-circuit, it causes more quickly the appearance of the first seed of liquid MCP2 material regardless of the thermal resistance formed by the thermoelement, without accelerating the complete melting of all the MCP2 material . As soon as the first seed of liquid of the material MCP2 appears, the material MCP2 is at the temperature T _f2 until the last seed of solid MCP2 material disappears. Therefore, the period during which the two MCP1, MCP2 materials are at constant temperatures is lengthened.

This prevents any or all of the MCP1 material from melting before the MCP2 material melts. This is achieved by bypassing the thermal resistance formed by the thermoelement.

These thermal short-circuit means make it possible to rapidly create a liquid seed of the MCP2 material without reducing the volume of MCP2 material involved in creating the thermal gradient, since the materials used in the conduit are added and do not participate in all. the cases at the creation of the thermal gradient. This amount of material is not "seen" by the thermoelement.

It is understood that the different ducts could have different diameters from each other, so the complete melting in the ducts having the largest diameters would be later than in the ducts having the smaller diameters. It is also possible to envisage one or more ducts with a variable diameter. This variant makes it possible to have a system with a scalable response: with increasing duct section, it will thus be possible to short-circuit higher powers or source times (see the data in the table above) for the same device.

On the Figures 4A to 4C we can see a variant of the short circuit means between the two speakers. In this variant, volume 28 entered two materials is partially filled with a liquid 30, for example water. This liquid is not miscible with either MCP1 material or MCP2 material. The volume of liquid 30, because of its deformability, provides a good interface between the molten material MCP1 and the MCP2 material not yet melted.

The operation of these short-circuit means is similar to that of means represented on the FIGS. 3A to 3C and will not be repeated here.

Advantageously, the thermogenerator according to the present invention also comprises means for delaying the complete solidification of the material MCP1, which is the material having the highest melting temperature Tf ₁ .

The means for retarding the complete solidification of the MCP1 material are formed by a zone containing MCP1 material whose dimensions, in particular the dimension between two inner surfaces of the enclosure is increased relative to the rest of the enclosure.

For example, in the case of the thermogenerator of the Figure 2A it can be provided that the zigzag cavity comprises an area in which the distance between two fins of the two parts 18, 20 defining a channel is greater than the characteristic dimension Lc, for example doubled or more. It can be provided in a variant that the enclosure comprises an additional reported tank, whose transverse dimension is larger than the characteristic dimension.

The ratio between the volume of the portion of material MCP1 having a thickness greater than the distance from the melting front and the volume of total MCP1 material is advantageously less than 30%, preferably less than 20%, more preferably of the order from 2 to 15%.

We will now explain the operation of this thermogenerator, considering that the MCP1, MCP2 materials are originally liquid.

In the absence of a heat source, the speakers cool, and indeed the fins 22 too, which causes a temperature drop of MCP1 and MCP2 materials.

The MCP1 material being the one with the highest melting temperature, it solidifies first.

As long as the solidification of the material MCP1 takes place and the liquid phase exists, the temperature of the material MCP1 is at the temperature T _f1 . The temperature gradient across the thermoelement is therefore constant. By providing this area of increased dimension, the solidification of the MCP1 material in this area is delayed relative to the rest of the MCP1 material. It is possible to extend the existence of a liquid phase of the material MCP1 and thus maintain a constant temperature T _f1 , and thus maintain a constant temperature gradient.

On the figure 5 it is possible to see schematically another mode of operation of a themogénératèur according to the present invention associated with a fluidic circuit.

Unlike thermogenerators Figures 1 and 2A the heat source does not directly irradiate the first phase change material element, but exchanges heat with a fluid circuit 32 in which a coolant circulates.

The heat source is located downstream of the second phase change material element 6 and upstream of the phase change material element 4 in the flow direction of the heat transfer fluid. The fluid circuit 32 forms a closed loop and passes through the second phase-change material element 6, the heat source 15 and the first phase-change material element 4. The heat-transfer fluid has a temperature at least equal to the temperature of the phase change of MCP2 material.

The enclosures of the phase change elements are such as to define cavities for the MCP1 and MCP2 phase change materials and channels for the coolant circulation. For example, tubes can pass through the enclosures of the phase change material elements 4, 6 in a sealed manner.

The hot source is for example an integrated circuit in a computer.

We will now explain how the circuit of the figure 5 .

The "cold" coolant passes through the second phase change material element and causes partial melting of the MCP2 material. Then, it passes through the hot source in which the coolant is heated to a water temperature less than T _f1 . The Heat transfer medium then passes through the first phase change material element causing partial melting of the MCP1 material. The coolant is then cooled and returned to the second phase change material element.

As long as the coolant circulates in the circuit, the MCP1 and MCP2 materials are in transition from solid-liquid phase. The temperature of each of the phase change material elements is constant, and therefore the temperature gradient applied to the thermoelement. The thermogenerator then generates a stable electrical power.

The system of figure 5 is particularly suitable for cooling of integrated circuits of a computer, the heat thus extracted allowing the generation of electricity that can be used by the computer. As long as the computer is operating, the coolant flows in the circuit 32 and keeps the MCP1, MCP2 materials in a transient state of phase change.

Due to the reversible behavior of phase change materials, as can be seen in the figure 6 , it is possible, thanks to the invention to obtain a generation of electrical energy that is stable cyclically as a function of the cyclic operation of the heat source: alternating periods of heat emission (heating and melting of the materials MCP1 and MCP2) and heat non-emission periods (cooling and solidification of MCP1 and MCP2 materials).

It is understood that the thermogenerator according to the invention may comprise several thermoelements electrically connected in series or in parallel depending on the application. Thermoelements are thermally connected in parallel. The thermoelements are arranged next to one another between the two phase change material elements.

The phase change materials that can be used in the present invention can be organic materials such as Rubitherm® RT100® whose phase change temperature is 99 ° C, benzoic acid whose temperature of change of the phase is 122 ° C, benzamide whose phase change temperature is 130 ° C, stilbene whose phase change temperature is 123 ° C, erythritol whose phase change temperature is 118 ° C,. ..), salt hydrates such as MgCl ₂ .6H ₂ O whose phase change temperature is 117 ° C, salts such as KNO ₃ -NaNO ₂ -NaNO ₃ whose phase change temperature is 140 ° C. ° C., the NaNO ₃ -KNO ₃ whose phase change temperature is 222 ° C., or metals such as Sn whose phase change temperature is 232 ° C.

Preferably, couples of phase change materials MCP1 and MCP2 will be selected having a significant difference in temperature, which will be advantageous for the generated electrical power.

Claims (15)

1. A thermogenerator comprising at least one thermoelement (2) and two phase-change materials (MCP1, MCP2) having different phase-change temperatures (T_f1, T_f2), said at least one thermoelement (2) having two opposite main faces (8, 10), and each of said faces (8, 10) being covered by one of the phase-change materials (MCP1, MCP2), such that the thermoelement (2) is subject only to the temperature gradient imposed by the two phase-change materials (MCP1, MCP2), during a phase of heating or cooling.
2. A thermogenerator according to claim 1, comprising enclosures containing the phase-change materials (MCP1, MCP2), each enclosure having a first and a second part (18, 20), the first part (18) comprising a flat plate (18.1), one face of which is fitted with protruding fins (22), and the second part (20) comprising a flat plane (20.1), one face of which is in contact with the thermoelement (2), and the other face of which is fitted with protruding fins (22), the first (18) and second (20) parts being installed facing one another, such that the fins (22) interpenetrate, defining a cavity (24) in which the phase-change material (MCP1, MCP2) is present.
3. A thermogenerator according to claim 2, in which the thickness of the phase-change material in at least a proportion of each of the enclosures is of the order of the melt front distance, the melt front distance being equal to $\sqrt{\frac{2.k\mathrm{.}\mathrm{\Delta }T\mathrm{.}t}{L}},$

   

   where k is the thermal conduction of the phase-change material, L is the latent heat of fusion of the phase-change material, ΔT is the temperature difference between the temperature of the wall of the fin and the phase-change temperature of the phase-change material, and t is the time.
4. A thermogenerator according to one of the claims 1 to 3, comprising means (16) to bring the two phase-change materials (MCP1, MCP2) into direct contact.
5. A thermogenerator according to claim 4, in which the means (16) to put the two phase-change materials into direct contact comprise at least one duct (26) connecting an enclosure in which the first phase-change material (MCP1) is present and an enclosure in which the second phase-change material (MCP2) is present, said duct (26) being partially filled by the first phase-change material and by the second phase-change material.
6. A thermogenerator according to claim 5, in which the transverse dimension of the duct (26) is less than or equal to the melt front distance, where the melt front distance is equal to $\sqrt{\frac{2.k\mathrm{.}\mathrm{\Delta }T\mathrm{.}t}{L}},$

   

   where k is the thermal conduction of the phase-change material, L is the latent heat of fusion of the phase-change material, ΔT is the temperature difference between the temperature of the wall of the duct and the phase-change temperature of the phase-change material, and t is the time.
7. A thermogenerator according to claim 5 or 6, in which at least one volume of liquid (30) separates the first and second phase-change materials in said duct.
8. A thermogenerator according to one of the previous claims, comprising enclosures containing the phase-change materials (MCP1, MCP2), the enclosure containing the phase-change material (MCP1) having the higher phase-change temperature (T_f1) comprising an area of phase-change material the thickness of which is greater than the melt front distance.
9. A thermogenerator according to the previous claim in combination with claim 2, in which the distance between two fins (22) is locally greater than the melt front distance.
10. A thermogenerator according to claim 8, in which said enclosure comprises an added tank containing phase-change material (MCP1), the transverse dimension of said tank being greater than the melt front distance.
11. A thermogenerator according to one of the previous claims, surrounded by thermal insulation means (17) to guide the heat flux through the stack formed by the phase-change materials (MCP1, MCP2) and the at least one thermoelement (2).
12. An electricity generating system comprising a thermogenerator according to one of the previous claims and a heat source (15).
13. An electricity generating system according to the previous claim, in which the heat source (15) is positioned on the side of the enclosure containing the phase-change material (MCP1) having the higher phase-change temperature (T_f1).
14. An electricity generating system according to claim 12, comprising a closed fluid circuit (32), containing a heat-transfer fluid, said circuit being able to exchange heat with the first and second phase-change materials (MCP1, MCP2), and traversing the heat source (15), in which the heat source (15) is located downstream from the phase-change material (MCP2) having the lower phase-change temperature (T_f2), and upstream from the phase-change material (MCP1) having the higher phase-change temperature (T_f1) in the direction of flow of the heat transfer fluid, and in which, when the heat transfer fluid exchanges with the phase-change material (MCP2) having the lower phase-change temperature (T_f2), its temperature is at least equal to the phase-change temperature (T_f2) of said material (MCP2), and when the heat transfer fluid exchanges heat with the phase-change material (MCP1) having the higher phase-change temperature (T_f1), its temperature is at least equal to the phase-change temperature (T_f1) of said material (MCP1).
15. An electricity generating system according to claim 12, 13 or 14, in which the heat source (15) is formed by at least one integrated circuit.

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